Fix mask slicing for models with HybridCache (#35681)
* correctly slice
* check mask
* Update modular_gemma2.py
* fix
* add tests
* fix typo
* finally fix mask slicing
* Finally correctly slice in all cases!!
* add test for all attention functions
* small fix in tests
* trick around dynamo tracing issue
* last update
* more robust
* kwargs propagation
* make it explicit for checkpointing
* apply modular
